This invention relates to a Doppler radar system, i.e. a radar system which derives information by observing the Doppler frequency shift of electromagnetic or other signals received after reflection from an object, there being relative motion between that object and an antenna of the radar system.
FIG. 1 depicts schematically an airborne radar system 1 travelling horizontally at velocity V over an area of land 2 and water 3. (In other applications the radar system could be carried by a space-craft and be travelling over the surface of the earth or an extraterrestrial body.) The radar system 1 has a transmit/receive antenna (not shown) whose boresight 4 is aligned with a point 5 on the ground. However, signals are received, at any one instant, from points spread over a finite area of the earth's surface. The amplitude components of the received signal from respective different points on the surface vary according to the polar diagram of the antenna. FIG. 1 illustrates schematically the main lobe and the two adjacent side lobes of the antenna, the beamwidth being grossly exaggerated on FIG. 1 for the purposes of explanation.
Thus, at the instant depicted in FIG. 1, the received signal has a component derived from a reflection at the point 6 on the sea surface immediately beneath the radar, from point 7 on the water/land boundary, and of course from the point 5 on the boresight of the antenna. Signal components received from these different points have different Doppler frequencies and it is desirable to establish what Doppler frequency is associated with the signal components received from point 5 on the antenna boresight. The frequency associated with such points on the antenna boresight is called the "center frequency." Having established the center frequency it is possible to filter from the received signal the component arising from reflections solely from the boresight of the antenna. This is useful for ground mapping purposes and in systems for measuring the velocity of an aircraft relative to the ground. It gives the radar good resolution.
If it could be assumed that all parts of the surface 2 had the same reflection properties it would be a simple matter to find the center frequency since this would be the frequency at which the received signal is strongest. The relationship of the Doppler frequency to the power of the components of the received signals having that Doppler frequency would be as shown in FIG. 2, the shape of this function depending entirely on the gain characteristics of the antenna. It should be noted from FIG. 2 that the center frequency f.sub.5 is coincident with the point of maximum amplitude of the received signal.
However, the amplitude of any particular frequency component of the received signal will be dependent on the reflection properties of the associated point on the surface 2 or 3 and noise. Thus, if the noise were constant for all frequencies, the theoretical relationship of FIG. 2 would be modified as shown in FIG. 3. Here there can be seen a discontinuity at frequency f.sub.7 caused by the water/land boundary at 7. This discontinuity displaces the maximum of the curve to the position f.sub.7 and so it could be assumed incorrectly that f.sub.7 is the center frequency. In a mapping radar this assumption would lead to inaccuracies at the very times when features of special significance are being observed. An object of this invention is to overcome this problem.